Oil well country tubular goods (OCTG) such as tubing and casing used in the excavation of oil wells are steel pipes having a length of around 10 some meters in which threads are formed on each end, and they are assembled to a necessary length by successively connecting the steel pipes in the field by means of threaded joints for pipes. Such threaded joints for pipes generally have a pin-box structure using a pin, which is a joint element having male threads, and a box, which is a corresponding joint element having female threads.
In a coupling-type threaded joint which is typically used for connecting oil country tubular goods, a pin is formed on the outer surface of both ends of a steel pipe which forms an oil country tubular good, and a box is formed on the inner surface of both sides of a short separate connecting member referred to as a coupling. There is a different type of threaded joint for pipes referred to as an integral type in which a pin is formed on the outer surface of one end of a steel pipe and a box is formed on the inner surface of the other end of the steel pipe and steel pipes are connected without using a coupling.
The depth of an oil well is normally 2,000-3,000 meters, but in recent deep wells such as undersea oil fields, the depth can reach 8,000-10,000 meters or more.
Therefore, a threaded joint for pipes used to connect oil country tubular goods is subjected in the environment of use not only to a tensile force in the pipe axial direction caused by the weight of oil country tubular goods and couplings but also to the combination of internal and external pressures and heat. As a result, the joint needs to be able to maintain gas tightness without being damaged even under a severe environment of use. During the process of lowering tubing or casing, a joint which has previously been tightened is sometimes loosened and retightened. API (American Petroleum Institute) standards require that tightening (makeup) and loosening (breakout) can be performed 10 times for a joint for tubing and 3 times for a joint for casing while maintaining gas tightness without the occurrence of non-repairable seizing referred to as galling.
A threaded joint for pipes referred to as a premium joint can form a metal-to-metal seal to provide improved gas tightness even under high stresses. In a premium joint, both the pin and the box have unthreaded metal contact portions in addition to threads. The unthreaded metal contact portions of the pin and the box directly contact each other to form a metal-to-metal seal whereby excellent gas tightness is exhibited. The unthreaded metal contact portion of the pin is constituted by a metal sealing surface positioned closer to the end of the pin than the threads on the outer peripheral surface of the pin and by a torque shoulder at the end of the pin. A corresponding metal sealing surface and torque shoulder are provided on the inner peripheral surface of the box. When the pin is inserted into the box and the threads are tightened until the torque shoulders contact each other, the metal sealing surfaces of the pin and the box intimately contact each other with a predetermined interference and form a metal-to-metal seal. A portion of the compressive load due to tightening is resisted by the torque shoulders which contact each other, whereby stress on the threads is decreased.
In a premium joint, it is important to provide sufficient lubricity to prevent galling because galling easily takes place on the unthreaded metal contact portions and particularly on the metal sealing surfaces. Until now, a highly viscous lubricating grease often referred to as dope or compound grease (referred to below as dope) has been applied to the surfaces of a pin and a box which contact each other at the time of makeup including the threads and unthreaded metal contact portions of a threaded joint for oil country tubular goods (referred to below simply as the contact surfaces of a threaded joint) prior to shipment with the object of increasing galling resistance and gas tightness and to prevent the contact surfaces from rusting until the time of use.
In the case of a coupling-type threaded joint for oil country tubular goods, in order to increase the circularity and shape accuracy of the end surface of a long steel pipe and to prevent the fluid which is flowing through the pipe from being disturbed by the connecting portion of the joint, a portion referred to as a recess or a chamfer which is cut into the shape of a taper has often been provided on the inner surface of the end of the pipe constituting a pin. Dope is also applied to the recess formed on the inner surface of a pin in order to prevent rusting.
In this manner, in the end portion of a steel pipe forming a pin, dope is applied not only to the outer surface and to the end surface, which are contact surfaces with a box, but it is also applied to the inner surface where a recess is formed. In order to guarantee lubricating properties and rust preventing properties (corrosion resistance), dope which was used in the past contained a large amount of powders of heavy metals such as Pb and Zn. Dope was usually applied by brush application, more specifically by dropping a suitable amount of dope onto the contact surfaces of a threaded joint and then spreading it with a brush.
Below-identified Patent Documents 1 and 2 disclose apparatuses for applying a highly viscous liquid lubricant like the above-described dope to the threads of a threaded joint for pipes which have a nozzle head for spraying a lubricant and a brush for spreading the sprayed lubricant.
With the enactment in 1998 of the OSPAR Convention (Oslo-Paris Convention) for preventing marine pollution in the Northeast Atlantic, strict environmental regulations are being enacted on a global scale, and particularly in North Sea oil fields, it is required to use lubricants not containing heavy metals with the object of preventing marine pollution. In order to minimize the discharge of substances causing marine pollution in the excavation of gas wells or oil wells on offshore rigs, the environmental impact of materials used on rigs which have the possibility of being discharged into the environment is being evaluated, and there is a trend of prohibiting the use of substances which do not satisfy the standards of a particular country or region. As a result, there have been various proposals of methods of lubricating and preventing rusting of threaded joints for pipes without requiring the application of dope.
Below-identified Patent Document 3 proposes a threaded joint for pipes having a box surface with a solid lubricating coating having plastic or viscoplastic Theological properties such as one made from a hot melt resin, and a pin surface having a solid corrosion protecting coating made from a UV curable resin. The thickness of the solid corrosion protecting coating made from a UV curable resin is preferably in the range of 5-50 μm and more preferably in the range of 10-40 μm.
In the threaded joint for pipes described in Patent Document 3, a UV cured resin coating is formed on the outer surface of an end portion of a steel pipe forming a pin. A UV cured resin coating is hard and its effect on prevention of corrosion is high. In addition, it is highly transparent, so it has the advantage that it is possible to inspect for the presence of damage on the threads of the pin, which is formed on the outer surface of a pipe and easily undergoes damage, without removing the coating. The solid lubricating coating formed on the surface of the box can flow when subjected to a high pressure. Therefore, even though the solid lubricating coating is applied only to the box surface, excellent lubricating properties which can prevent galling of a threaded joint for pipes are obtained during repeated makeup and breakout. The UV cured resin coating on the pin surface and the solid lubricating coating on the box surface both are non-tacky coatings (referred to as dry coatings), and they prevent the occurrence of galling due to the adhesion of foreign matter which occurs with dope.